Minerals with mixed valence states are widespread and form in many different rock types. They can contain, for example, Fe2+ - Fe3+ and Mn2+ - Mn3+ - Mn4+, with the ratios of oxidation states reflecting the redox conditions under which the host materials crystallized. The distribution of the ratio of iron (III) to total iron content (Fe3+/ΣFe) in minerals reflects the oxidation states of their host rocks and is therefore important for answering fundamental questions about the Earth's evolution and structure. Iron is the most sensitive and abundant indicator of oxidation state, but many mineral samples are too fine-grained and heterogeneous to be studied by standard methods such as Mossbauer spectroscopy, electron microprobe, and wet chemistry. Here we report on the use of electron energy-loss spectroscopy with a transmission electron microscope to determine Fe3+/ΣFe in minerals at the nanometre scale. This procedure is efficient for determining Fe3+/ΣFe ratios of minor and major amounts of iron on a scale heretofore impossible and allows information to be obtained not only from ultra-fine grains but also, for example, at reaction fronts in minerals.
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